The equation of time is a parameter used in astronomy to report the relative visible movement of the Sun relative to the average sun, which can differ from one another by plus or minus approximately 15 minutes over the course of a year. From one year to the next, the annual evolution curve of this parameter repeats itself practically identically.
The average solar time is based on the average sun, defined as an object which moves over the equator at a constant speed throughout the year, such that the length of the average solar day is exactly 24 hours.
The solar time, or true time, is a measurement of the time based on the true sun, as given by a sundial. In particular, solar noon corresponds to the point of the day where the Sun reaches its highest point in the sky.
By convention, the equation of time is the quantity that must be added or removed each day to go from the true time to the average time, and corresponds, at a given moment, to the difference between the average solar time and the true solar time.
A positive value of the equation of time indicates that the true sun is behind relative to the average sun, i.e., more to the east, and a negative value indicates that it is in advance, i.e., further to the west. For example, when the time equation is equal to +8 minutes, this means that it is 12:08 in average solar time when the sundial indicates true noon.
There are already different display modes for the time equation that generally use a hand commanded by a cam of the equation of time provided inside a clockwork movement, said hand moving along fixed graduations displayed on the dial, to make it possible to display the value of the equation of time. Such a complication is for example shown diagrammatically by I. Vardi “La marche du soleil—Un affichage naturel de l'érequation du temps”, Bulletin of the Société suisse de Chronométrie, SSC, Neuchâtel, CH, no. 62, Dec. 1, 2009, pages 37-44. However, according to this construction, the hand indicating the equation of time does not provide any tendency of the evolution of the equation of time for future months. The user does not know whether the difference between the average solar time and the true solar time will decrease, or on the contrary, increase.
One aim of the present invention is to propose a timepiece, such as a bracelet watch, allowing a dynamic and original new display of the equation of time.